CAP1/CAP2 at the Actin Filament Barbed End: Reconciling Kinetic, Single-Molecule, Structural and Cellular Evidence

Cyclase-associated proteins CAP1 and CAP2 regulate actin monomer recycling and cooperate with cofilin at filament pointed ends. Recent reconstitution studies show that the C-terminal region also accelerates barbed-end depolymerization, but the underlying mechanism remains unsettled. A central unresolved issue is whether the CARP domain contributes directly. Published constructs change CARP content together with scaffold architecture or valency, so they do not isolate this variable. We therefore specify matched construct series that would test CARP contribution directly. We also evaluate conflicting formin results and the evidence for processivity, with explicit attention to labeling, surface effects, filament state, accessory proteins, and species or construct differences. The evidence strongly supports C-CAP activity and WH2 involvement, whereas CARP contribution, the basis of the formin discrepancy, and mammalian CAP processivity require further testing. Structural and high-speed atomic force microscopy studies narrow the possible mechanisms but do not independently establish C-CAP engagement at the barbed end, and cellular studies have not directly measured C-CAP occupancy or activity at barbed ends. Monomer availability, filament nucleotide state, barbed-end occupancy, and local CAP abundance are therefore treated as candidate variables for prospective cellular tests. Tropomyosin decoration is an additional hypothesis rather than an established determinant.